<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .nav {
            height: 30px;
            background: #036663;
            border-bottom: 1px solid #369;
            line-height: 30px;
            padding-left: 30px;
        }

        .nav a {
            color: #fff;
            text-align: center;
            font-size: 14px;
            text-decoration: none;

        }

        .d-box {
            width: 400px;
            height: 300px;
            border: 5px solid #eee;
            box-shadow: 2px 2px 2px 2px #666;
            position: absolute;
            top: 40%;
            left: 40%;
            background-color: white;

            /* 不让文字被选中 */
            -webkit-user-select:none;
            -moz-user-select:none;
            -ms-user-select:none;
            user-select:none;
        }

        .hd {
            width: 100%;
            height: 25px;
            background-color: #7c9299;
            border-bottom: 1px solid #369;
            line-height: 25px;
            color: white;
            cursor: move;
        }

        #box_close {
            float: right;
            cursor: pointer;
        }
    </style>
</head>
<body>
<div class="nav">
    <a href="javascript:;" id="register">注册信息</a>
</div>
<div class="d-box" id="d_box">
    <div class="hd" id="drop">注册信息 (可以拖拽)
        <span id="box_close">【关闭】</span>
    </div>
    <div class="bd"></div>
</div>
<script>
    var drop=document.getElementById("drop");
    var box=document.getElementById("d_box");
    var box_close=document.getElementById("box_close");
    drop.onmousedown= function (e) {
        e=e||window.event;
        var x= e.pageX-box.offsetLeft;
        var y= e.pageY-box.offsetTop;

        document.onmousemove=function(e){
            e=e||window.event;
            var boxX= e.pageX-x;
            var boxY= e.pageY-y;

            box.style.left=boxX+"px";
            box.style.top=boxY+"px";

            if(box.offsetLeft<=0){
                box.style.left="0px";
            }
            if(box.offsetTop<=0){
                box.style.top="0px";
            }
            var width=window.screen.width-box.offsetWidth;
            var height=window.screen.height-box.offsetHeight;
            if(box.offsetLeft>=width){
                box.style.left=width+"px";
            }
            if(box.offsetTop>=height){
                box.style.top=height+"px";
            }
        }
    }
    drop.onmouseup=function(){
        document.onmousemove=null;
    }
    box_close.onclick=function(){
        box.style.display="none";
    }
</script>
</body>
</html>